The present invention relates generally to the field of optical transmission systems, and more specifically to a dense wavelength division multiplexed optical transmission system providing enhanced control of end-to-end channel performance to extend fiber transmission distance.
Dense Wavelength Division Multiplexed (DWDM) optical transmission systems have been widely deployed in optical networks to increase network speed and capacity. A conventional DWDM optical transmission system comprises a plurality of optical transmitters configured to transmit respective channels of information at different wavelengths, an optical multiplexor configured to combine the respective channels into a multi-wavelength optical signal for transmission on a single transmission fiber, a plurality of serially connected optical amplifiers configured as repeaters to amplify the multi-wavelength optical signal at intervals along a transmission path, an optical de-multiplexor configured to separate the multi-wavelength optical signal into its component channels, and a plurality of optical receivers configured to receive and detect the information carried by the respective channels.
One drawback of the conventional DWDM optical transmission system is that the optical amplifiers disposed along the transmission path typically have wavelength dependent gain and noise profiles, which can cause unequal channel performance. For example, the performance of the channels in the conventional DWDM optical transmission system may be characterized by associated Optical Signal-to-Noise Ratio (OSNR) values. Further, because of the wavelength dependent gain and noise profiles of the optical amplifiers, the OSNR values associated with the channels may not be equal to one another at the receiver end of the transmission path even though the channels may have the same optical power levels at the transmitter end of the path.
One approach to compensating for such unequal channel performance in the conventional DWDM optical transmission system is to perform a pre-emphasis technique at the transmitter end of the transmission path. For example, the OSNR values of the respective channels may be monitored at the receiver end of the transmission path by a measurement device such as an optical spectrum analyzer, and the pre-emphasis attenuation or gain of the respective channels may be adjusted by varying the optical power levels at the transmitter end of the path based on the measured OSNR values to achieve designated OSNR values at the receiver end of the path.
However, performing such pre-emphasis techniques at the transmitter end of the transmission path to compensate for unequal channel performance at the receiver end of the path has its own drawbacks. For example, pre-emphasis techniques that raise optical power levels of selected channels at the transmitter end of the transmission path may increase power levels at the optical amplifier outputs, which may in turn increase the total power requirements of the optical transmission system. Having high power levels in some channels may also introduce transmission impairment due to fiber non-linearity, especially for channel bit rates of 10 Gbit/s or more. Therefore, such pre-emphasis techniques are typically only used to compensate for unequal channel performance over a limited fiber transmission distance.
Another approach to equalizing channel performance in the conventional DWDM optical transmission system, which may be used in conjunction with the above-mentioned pre-emphasis technique, is to terminate and regenerate the multi-wavelength optical signal on the transmission path. However, this approach also has drawbacks in that such optical signal termination/regeneration typically requires optical-to-electrical and electrical-to-optical conversions, which are very costly and usually must be performed at regular intervals along the transmission path.
It would therefore be desirable to have a DWDM optical transmission system that compensates for unequal channel performance. Such a DWDM optical transmission system would be capable of compensating for unequal channel performance over an extended fiber transmission distance, thereby reducing the need for terminating and regenerating multi-wavelength optical signals on an optical transmission path.
In accordance with the present invention, a Dense Wavelength Division Multiplexed (DWDM) optical transmission system is provided that compensates for unequal channel performance over an extended fiber transmission distance. The presently disclosed invention achieves such benefits by way of a multiple pre-emphases technique that provides enhanced control of the channel performance from a transmitter end to a receiver end of an optical transmission path.
In one embodiment, the DWDM optical transmission system includes a plurality of optical transmitters at a transmitter end of a transmission path configured to transmit respective channels of information at different wavelengths, a first pre-emphasis device configured to perform a first pre-emphasis technique on the respective channels, an optical multiplexor configured to combine the respective channels into a multi-wavelength optical signal for transmission on a single transmission fiber, at least one optical amplifier configured to amplify the multi-wavelength optical signal along the path, at least one second pre-emphasis device disposed along the path and configured to perform a second pre-emphasis technique on the respective channels, an optical de-multiplexor configured to separate the multi-wavelength optical signal into its component channels, and a plurality of optical receivers at a receiver end of the path configured to receive and detect the information carried by the respective channels.
The first pre-emphasis technique performed by the first pre-emphasis device includes measuring a first plurality of Optical Signal-to-Noise Ratio (OSNR) values of the respective channels at an output of the second pre-emphasis device, and adjusting pre-emphasis attenuation and gain of the respective channels based on the first measured OSNR values to achieve a first plurality of designated OSNR values of the respective channels at the second pre-emphasis device output. In the first pre-emphasis technique, the OSNR values of the respective channels are measured by a measurement device such as an optical spectrum analyzer.
The second pre-emphasis technique performed by the second pre-emphasis device includes measuring a second plurality of OSNR values of the respective channels at the receiver end of the transmission path, and adjusting pre-emphasis attenuation and gain of the respective channels based on the second measured OSNR values to achieve a second plurality of designated OSNR values of the respective channels at the receiver end of the path.
In the second pre-emphasis technique, the OSNR values of the respective channels are measured by one of a plurality of possible OSNR measurement techniques. Each OSNR measurement technique takes into account Amplified Spontaneous Emission (ASE) noise at the output of the second pre-emphasis device, which may have been modified by dispersion components included in the second pre-emphasis device. A first OSNR measurement technique includes measuring out-of-band ASE noise levels of the respective channels at the second pre-emphasis device output, and estimating actual ASE noise levels of the respective channels using the measured out-of-band ASE noise. A second OSNR measurement technique includes measuring in-band ASE noise levels of the respective channels at the second pre-emphasis device output by alternately turning optical signal power xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d. A third OSNR measurement technique includes measuring optical signal power levels of the respective channels at the second pre-emphasis device output by dithering optical signal carriers, and estimating OSNR values at the respective channels using the measured optical signal power.
By employing a plurality of pre-emphasis devices to perform a multiple pre-emphases technique on the respective channels of information, the DWDM optical transmission system compensates for unequal channel performance while reducing the need for terminating and regenerating the multi-wavelength optical signal on the optical transmission path.